Switching power supplies of this type are applied e.g. for supplying electronic devices installed in industrial measurements technology, e.g. electronic devices such as measuring devices, sensors, measuring- and/or control circuits or signal processors, with an output voltage supplied by the switching power supply and controlled to a predetermined, desired value.
Switching power supplies for producing an output voltage controlled to a predetermined, desired value regularly comprise a supply path and a feedback path. The supply path serves to receive the electrical energy supplied to switching power supply on the input side, to convert the electrical energy and to supply it via an output of the switching power supply. The feedback path serves for leading a signal reflecting the output voltage, applied to the output, back to the input side, based on which the output voltage is controlled to the predetermined desired value by means of a control provided on the input side.
There are a large number of applications, in the case of which controlled switching power supplies are applied for supplying consumers in explosion endangered regions. In explosion endangered regions, strict safety specifications rule, which, among other things, have the goal to prevent a spark formation, which in given cases could trigger an explosion. Beyond that, there are allowed in explosion endangered regions, as a rule, only switching power supplies, whose primary side feedable via an external energy source is galvanically isolated from the secondary side supplying the consumer. In the case of controlled switching power supplies, there is, thus, both in the supply path as well as also in the feedback path, in each case, a galvanic isolation, which meets the explosion protection safety specifications ruling at the location of use.
Corresponding safety requirements are to be found e.g. in the standard IEC 60079-11 published by the International Electrotechnical Commission (IEC), version of Jan. 27, 2012. There are e.g. in part 11 specifications for galvanic isolations meeting the isolation requirements of the explosion protection class, intrinsic safety (Ex-i). According to that, an isolation sufficient for this protection class can be effected via a fixed insulation, which for applications in a voltage range up to 60 V DC has to be at least 0.5 mm thick, and in a voltage range up to 230 V AC at least 1 mm thick. Alternatively, also an air gap can be used, which for applications in a voltage range up to 60 V DC has to be at least 3 mm wide, and in a voltage range up to 230 V AC at least 10 mm wide.
For maintaining these specifications, optocouplers with a correspondingly large insulation resistance can be applied in the feedback path. Commercially obtainable optocouplers, which satisfy these requirements, are, however, quite expensive, even in the case of orders at large numbers.
A significantly cost effective alternative is provided by a signal transmitter comprising a light source and a separate light receiver transforming light striking thereon into an electrical variable. Thus, described, for example, in DE 10 2010 012 064 A1, is a discretely constructed signal transmitter, which includes a light source, e.g. an infrared light emitting diode, arranged on one side of a circuit board and sending light through the circuit board that is received then by a light receiver arranged on the other side of the circuit board.
Conventional light sources, such as e.g. infrared light emitting diodes, usable for optical signal transmission must be operated, however, only with a supply voltage, which, as a rule, is significantly lower than output voltages usually required on the outputs of switching power supplies. For reduction of the voltage falling across the light source, the light source can be preceded by an appropriately sized limiting resistance.
Conventional light emitting diodes suitable for optical signal transmission must be operated with a comparatively high electrical current. Thus, infrared light emitting diodes, for example, must be operated with an electrical current in the order of magnitude of up to 50 mA, sometimes even up to 100 mA, in order to transmit sufficient light through a circuit board with a wall thickness sufficient for applications in explosion endangered regions or through an air gap similarly sufficient. That means that a correspondingly large power loss occurs in the limiting resistance. If a 15 volt decrease must occur across the limiting resistance, then there arises in the case of an electrical current in the order of magnitude of 50 mA to 100 mA a power loss in the order of magnitude of 0.75 W to 1.5 W.
If one would apply such an optical signal transmitter in the feedback path of a switching power supply, then the power loss occurring through the limiting resistance would decrease the efficiency of the switching power supply. Moreover, there is the problem that the power loss is converted into heat. In this way, depending on the size of the voltage falling across the limiting resistance and the electrical current required by the light source, temperatures can occur in the region of the limiting resistance, which make use of the signal transmitter impossible in explosion endangered regions.